1. Field of the Invention
The present invention relates to a Coherent Doppler Lidar (CDL), and more particularly to a Coherent Doppler Lidar a laser light source of which to be loaded therewith is improved.
2. Description of the Related Art
Recently, demands for aviation security build up rapidly with increase of users in flights due to developments in aircraft.
Particularly, aircraft accidents due to air turbulence in fair weather are frequently reported in these days, so that a proactive measure of such aircraft accidents is an urgent problem.
In this respect, however, the air turbulence in fair weather is not only a phenomenon which cannot be visually recognized as a matter of course, but also a phenomenon which cannot be recognized by the use of radar. As a consequence, it is pointed out that the avoidance of the air turbulence in fair weather is difficult, even at the present days where computer-controlled aeronautical systems are made up.
Under the circumstances, engineering developments for remote sensing being a technology for observing an observation target by applying radio waves or lights from a remote place are strongly desired as a technology by which a sudden phenomenon such as air turbulence in fair weather is recognized in advance to avoid a possible accident derived therefrom. In this connection, a coherent Doppler lidar being a coherent Doppler sensor which may be designed compactly so as to be loaded with an aircraft is watched at present.
The coherent Doppler lidar will be described herein. First, both the LIDAR: (Light Detection And Ranging) and RADAR: (Radio Detection And Ranging) differ from one another in that the radar transmits radio waves, while the lidar uses laser beams in place of radio waves as a transmission light source.
The coherent Doppler lidar detects coherently signals, and measures frequencies of beat signals with respect to a local oscillator, whereby a Doppler shift of the frequencies due to an observation target is highly accurately detected.
In a coherent Doppler lidar loaded with an aircraft, targets reflecting laser beams are fine particles (aerosol) in the atmosphere and clouds.
Next, the principle of operations of the above-described coherent Doppler lidar will be described by referring to FIG. 1 of a constitutional, explanatory block diagram showing a conventional coherent Doppler lidar. In the coherent Doppler lidar, a seed light output from a master laser 10 is subjected to injection-locking, whereby an oscillation wavelength of the pulse laser 12 is controlled.
Pulse laser beams output from the pulse laser the oscillation wavelength of which is thus controlled are projected into the atmosphere through an input/output system 14 such as a telescope, and a scanner.
A reflected light which is derived from the one in accordance with such a manner that the pulse laser beams projected into the atmosphere as described above are subjected to Doppler shift in response to the behavior of aerosol is received through an input/output system 14, the reflected light is mixed with the laser beams from the master laser 10, and synthesized by a mixer 16a of a detection system 16.
Low frequency beat signals among the signal components synthesized by the mixer 16 are amplified by an IF amplifier 16b, the amplified signals are analog/digital-converted in an A/D converter 16c, and the resulting converted signals are stored in a computer 16d as digital signals.
In the computer 16d, a Doppler shift is determined by means of frequency analysis and is converted to a wind speed after removing an offset component from the Doppler shift, and the results obtained are displayed on a display 16e or stored in a storage device 16f such as a HDD or a digital tape.
In order to easily understand the operations of the coherent Doppler lidar, although an explanation is omitted in the above description, there is also in an actual measurement such a procedure that a part of the output from the pulse laser 12 is mixed with the output of the master laser 10 to monitor offsets of the output laser as reference signals in every pulses.
Incidentally, a coherent Doppler lidar functions to emit pulse laser beams into the atmosphere as described above. In this connection, it is desired to use such pulse laser oscillating in a region of a wavelength longer than 1.5 μm being a so-called eye safe region which is highly safe with respect to human eyes with taking the emission of pulse laser beams into the atmosphere in remote sensing into consideration.
Thus, developments of such coherent Doppler lidar which can oscillate high-energy pulses, and output laser beams in a region of a wavelength longer than 1.5 μm into the atmosphere are strongly desired.